Could be in the last century, can be cowboy style
Does https://storiesonline.net/s/44553/lisa-marie-amp-unca-tom qualify? Premier only (and also available on his site under another name). Minimal sex content though, and unfinished.
https://storiesonline.net/s/46723/car-54 is by the same author.
This is the prequel to Split Tails Ranch.
https://storiesonline.net/s/22165/the-walters-brothers
Most of the rest of qhml1's work is also good
Although technically a ranch rather than a farm, you may want to check out "Uncle Randy and the Angry Niece" by Russell Hoisington. It's part of the Wynter/ Trailer Park universe, but works as a standalone.
https://storiesonline.net/s/58376/uncle-randy-and-the-angry-niece
Be aware that it is a Premier story, since Russell passed back in 2010, but it's worth it. In fact, the whole 14-story sequence of Russell's "Wynter" stories and Wizard's "Trailer Park" stories is quite good; the lowest score of the entire set is 7.91.
Keep in mind that the stories predate the under-14 rule, though.
The under-14 rule does not apply in that story, I think she's older than that.
It very much applies in the Wynter stories and I found the first one to be very difficult to read for exactly that reason.